Wavelength division multiplexed optical communication system having tunable multi-channel dispersion compensating filters

ABSTRACT

In accordance with the invention, a WDM optical communication system includes a new tunable multi-channel dispersion compensating filter having low loss, low polarization dependence and capable of compensating many channels over a large wavelength range. In essence, the filter comprises an optical cavity with a near 100% reflector on one side and a variable partial reflector on the other side. The device acts as a tunable all-pass filter.

FIELD OF THE INVENTION

[0001] This invention relates to optical communication systems and, inparticular to wavelength division multiplexed optical communicationsystems (WDM and dense WDM systems) having tunable multi-channeldispersion compensating filters.

BACKGROUND OF THE INVENTION

[0002] Optical communication systems are usually based on high puritysilica optical fiber as the transmission medium. Conventionalterrestrial systems are typically designed to transmit optical signalsin a wavelength range where longer wavelength components are subject toslightly longer propagation time delay than shorter wavelengths(positive chromatic dispersion). To prevent this dispersion fromdeteriorating the information content of the optical signals, earlysystems used a single channel at a wavelength where dispersion is low orzero.

[0003] As it has become desirable to utilize many channels over a widerrange of optical wavelengths (WDM systems), chromatic dispersion hasrequired more precise compensation. WDM systems are important for theirability to transmit vast amounts of information and for their ability toincorporate network functions such as add/drop and cross connecting. Butas the bit rate of WDM channnels increases, chromatic dispersioncompensation becomes critical.

[0004] Typically dispersion compensation schemes for WDM systems involvethe use of dispersion compensating fiber and dispersion compensatinggratings. The transmission fibers used in terrestrial systems typicallyexhibit net positive chromatic dispersion which, for WDM systems, cannotbe wholly compensated by dispersion fiber. Although segments of suchfiber can be used to compensate the accumulated dispersion in atransmission fiber span, optimum compensation is usually achieved onlyfor chosen channels (typically in the middle of the transmission band).There remains a residual wavelength dependent dispersion in channelslocated at the extremes of the transmission band due to the dispersionslope.

[0005] Compensating the accumulated dispersion of the extreme channelscan require a dispersion compensating grating (DCG). DCGs are chirpedfiber Bragg gratings used in reflection mode and oriented so that thelong wavelengths are reflected first before short wavelengths. In thismanner, optical pulses broadened due to the accumulated positivechromatic dispersion can be recompressed in time. Typical arrangementusing DCGs are described in U.S. Pat. No. 4,953,939 issued to R. E.Epworth on Sept. 4, 1990 and U.S. Pat. No. 5,701,188 issued to M.Shigematsu et al. on Dec. 23, 1997, both of which are incorporatedherein by reference. One of the main advantages of using DCGs is thatthe amount of dispersion and the dispersion slope can be easily adjustedby setting the grating chirp parameters. Another advantage is their lownon-linearity.

[0006] However conventional compensation schemes using dispersioncompensating fiber and DCGs present a number of shortcomings. Thedispersion compensating fibers typically introduce significant loss andrespond to the input signal power in a non-linear fashion. DCGs canintroduce polarization mode dispersion and, because they tend to belong, introduce group delay ripples that must be minimized. Accordinglythere is a need for a new WDM communication system providing low loss,low polarization dependent compensation over a wide bandwidth.

SUMMARY OF THE INVENTION

[0007] In accordance with the invention, a WDM optical communicationsystem includes a new tunable multi-channel dispersion compensatingfilter having low loss, low polarization dependence and the capabilityof compensating many channels over a large wavelength range. In essence,the filter comprises an optical cavity with a near 100% reflector on oneside and a variable partial reflector on the other side. The device actsas a tunable all-pass filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The nature, advantages and various additional features of theinvention will appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

[0009]FIG. 1 schematically illustrates a WDM optical fiber communicationsystem employing a tunable multi-channel dispersion compensating filterin accordance with the invention;

[0010]FIG. 2 illustrates a preferred embodiment of the dispersioncompensating filter used in the system of FIG. 1;

[0011]FIG. 3 shows an alternative form of the dispersion compensationfilter;

[0012]FIG. 4 is graphical illustrations useful in understanding theoperation of the system and filter.

[0013]FIG. 5 illustrates an alternative two-stage filter; and

[0014]FIGS. 6, 7 and 8 are graphical illustrations useful inunderstanding the FIG. 5 device.

[0015] It is to be understood that these drawings are for purposes ofillustrating the concepts of the invention and, except for the graphs,are not to scale.

DETAILED DESCRIPTION

[0016] Referring to the drawings, FIG. 1 schematically illustrates a WDMoptical fiber communication system 10 comprising a multiwavelengthoptical transmitter 11, a transmission fiber 12, and a tunablemulti-channel dispersion compensating filter 13 to compensate dispersionin fiber 12. It also includes one or more multiwavelength opticalreceivers 14. Optionally, depending on the length of the system, anoptical amplifier 15 can be disposed between the transmitter 11 and thereceiver 14. Long distance transmission systems may comprise a pluralityof segments of fiber 12 with respective segments interconnected byamplifiers and dispersion compensators including filters 13. Longsystems may also include one or more intermediate add/drop nodes (notshown) between successive segments of transmission fiber 12.

[0017] Conveniently, the filter 13, which is a reflection device, iscoupled to segments of fiber 12 by a dual fiber photonics ferrulepackage 16 such as that described in co-pending application Ser. No.08/688,178 filed by Fever et al. on Jul. 26, 1996 and assigned toapplicant's assignee

[0018] In operation, multiple wavelength signal channels at wavelengthsλ₁, λ₂, λ_(n), are launched by transmitter 11 down fiber 12. Thechannels incur loss and dispersion as they pass through the fiber. Thesignals can be amplified by amplifier 15 and dispersion compensated byfilter 13.

[0019]FIG. 2 illustrates a preferred embodiment of a tunablemulti-channel dispersion filter 13. In essence, the filter 13 is amodified form of the mechanical anti-reflection switch modulator (MARSmodulator) described by K. W. Goossen et al. in U.S. Pat. No. 5,500,761issued Mar. 19, 1996 which is incorporated herein by reference. Thefilter 13 is modified, as compared to the Goossen et al. device, by theinclusion of an optical cavity 27 of low loss optical material definedbetween a pair of parallel sides 28, 29. One side 29 is a fixed mirror(a near−100% reflector with reflectivity≧95%).

[0020] The device comprises a variable partial reflector 19, a fixedmirror 29 and an optical cavity 27 disposed in the light path betweenthem. The partial reflector 19 comprises a substrate body 20 having agenerally planar surface and a movable membrane 21 spaced generallyparallel to the surface to define on gap 22 between the membrane and thesubstrate. The substrate 20 can be crystalline silicon and the membrane21 can comprise polycrystalline silicon. The membrane 21 and thesubstrate 20 are spaced apart by a peripheral support layer 23 ofinsulating material. Electrodes 24, 25 permit connection of the membraneand substrate, respectively to the terminals of a controllable voltagesource 26.

[0021] The parallel sides 28, 29 define an optical cavity of thicknessL. The cavity thickness determines the filter's periodicity or freespectral range (FSR). The thickness L can be chosen so that the FSR isan integral multiple of the spacing between optical channels in thesystem. In this way, many channels can be compensated simultaneously.The thickness L can be tuned, as by thermal tuning. A change in thecavity optical path length of λ/2 translates the filter frequencyresponse by one complete FSR.

[0022] The spacing between membrane 21 and side 28 can be controlled byan applied voltage. When the distance between the membrane and substrateis an odd multiple of λ/4, the reflectance is a maximum. When thedistance is an even multiple of λ/4, the reflectance is a minimum. Ifdesired, the cavity length can be tuned, as by temperature tuning, tomaintain wavelength locking.

[0023]FIG. 3 shows an alternative form of a tunable multi-channeldispersion filter 13 wherein the variable partial reflector 19 and thefixed mirror 29 are disposed on two separate substrates 30, 31,respectfully, and the optical cavity 27 comprises the gap between 19 and29. Substrate 30 is advantageously transparent and preferably isprovided with an antireflection coating 32.

[0024]FIG. 4 is a graphical illustration showing the group delay of asignal from passing through the device of FIG. 2 for two differentmembrane deflections. As can be seen, the group delay of the signal iswavelength dependent. The peak delay increases as the partialreflectance increases.

[0025] The dispersion produced by the filter 13 is measured by thederivative of the filter delay with respect to wavelength. In manyapplications a constant dispersion is required. A composite filterhaving an increased passband width of nearly constant dispersion can bemade by concatenating a plurality of filters 13. If desired, the filterscan be offset by a controlled wavelength shift. Feedback control of thetuning can be used to obtain active stabilization.

[0026]FIG. 5 illustrates a composite filter 40 made by concatenating twofilters 13A and 13B. Each of the filters 13A, 13B has the structureshown in FIG. 2.

[0027]FIG. 6 shows the delay for a two stage filter of FIG. 5 over oneperiod (FSR). The dotted line, for comparison, shows an ideal lineardelay characteristic. The delay is given in units of T, defined as thetime for the signal to travel one roundtrip in the cavity. For a filterwith a FSR of 100 GHz, the roundtrip delay is T=10 ps. By varying thecavity reflectances and optical path lengths, the delay response ismodified to produce a range of constant dispersions.

[0028] In practical filters there is some loss. FIG. 7 illustrates theeffect of a typical loss of 0.65 dB loss per cavity roundtrip on thetwo-stage filters of FIG. 6. The loss varies approximately linearly overthe passband. Smaller filter loss variations with respect to wavelengthcould result from smaller cavity losses.

[0029] An important consideration for dispersion compensating filters isthe degree to which the desired dispersion response is achieved acrossthe passband. This achievement is measured by a figure of merit calledthe group delay ripple which calculates the deviation of the group delayfrom the desired linear response. FIG. 8 illustrates the group delayripple for the filter of FIG. 6. As can be seen from FIG. 8, thepeak-to-peak ripple is only about 0.05×T over the passband.

[0030] The passband width, dispersion, and ripple scale with the FSR.For the design example, the values for these parameters in standardunits for several FSRs are given in Table 1: TABLE 1 Design parametersfor different cavity thicknesses (different FSRs) assuming a passbandwidth equal to 40% of the FSR in each case. Ripple FSR (GHz) BW + 0.4(GHz) Dmax (ps/nm) T (ps) p-p (ps) 100 40 62.5 10 0.5 50 20 250 20 1 2510 1000 40 2 12.5 5 4000 80 4

[0031] For a 100 GHz FSR, the passband width is 40 GHz in this example.A maximum dispersion of 60 ps/nm is achieved with a ripple of less than0.5 ps. The ripple is very small compared to chirped Bragg gratingfilters, which can easily have 10 ps of ripple. The sign of the filterdispersion can easily be reversed by changing the relative cavitylengths of the two-stages, for example by temperature tuning. Thus, thefull tuning range is twice the peak dispersion.

[0032] It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be readily devised by thoseskilled in the art without departing from the spirit and scope of theinvention.

What is claimed is:
 1. In an optical fiber communication systemcomprising a wavelength division multiplexed optical transmitter, alength of optical transmission fiber for transmitting signals from thetransmitter, the fiber producing unwanted chromatic dispersioncompensator for reducing chromatic dispersion in the signals and anoptical receiver for the signals, the improvement wherein: thedispersion compensator comprises an all-pass filter comprising avariable reflectivity mirror, a fixed mirror, and an optical cavity inthe light path between the mirrors.
 2. The improved system of claim 1wherein the variable reflectivity mirror comprises a substrate having afirst planar surface, a membrane spaced parallel to the surface and acontrollable voltage source to vary the distance between the membraneand the surface.
 3. The improved system of claim 1 wherein thetransmitter transmits a plurality of spaced wavelength signal channelsand the optical cavity of the all-pass filter is dimensioned so that thefree spectral range of the filter is equal to the wavelength spacingbetween at least two signal channels.
 4. An all-pass optical filtercomprising: a variable reflectivity mirror comprising an electricallymovable membrane; a fixed mirror; an optical cavity between the variablereflectivity mirror and the fixed mirror; and a controllable voltagesource for moving the membrane.
 5. The all-pass filter of claim 4wherein the variable reflectivity mirror comprises a substrate having afirst planar surface, a membrane spaced parallel to the surface and acontrollable voltage source to vary the distance between the membraneand the surface.
 6. The all-pass filter of claim 5 wherein said opticalcavity comprises the substrate.
 7. The all-pass filter of claim 5wherein said substrate further comprises a second surface parallel tothe first, said second surface supporting the fixed mirror.
 8. Theall-pass filter of claim 5 wherein said substrate further comprises theoptical cavity.
 9. The all-pass filter of claim 5 wherein the variablereflectivity mirror and the fixed mirror are formed on separate spacedapart substrates.
 10. The all-pass filter of claim 9 wherein the opticalcavity comprises the space between the spaced apart substrates.